1. Field of the Invention
The present invention relates generally to an optical image forming method and device for forming a light image on an object, and more particularly to an optical image forming method and device for forming a light image on an object which moves in one direction relative to reflected light irradiated thereto from a spatial light modulator including a plurality of mirrors each capable of independently controlling the inclination of its reflecting surface, as well as to a printer, an image forming apparatus such as a copying machine, and an aligner for lithography, all of which may utilize the device.
2. Description of the Related Art
Conventionally, a device named a digital micromirror device (hereinafter referred to as the "DMD") is known as a spatial light modulator of the type mentioned above. Specifically, the DMD has a multiplicity of micromirrors, referred to as micromirrors, each rotatable about a fixed axis, formed on a semiconductor substrate of Si or the like. The DMD is irradiated with light from a light source which is deflected by each of the micromirrors, the rotation of which is controlled by an electrostatic action or the like. Each of the micromirrors is selectively rotated to change the inclination of its reflecting surface to selectively irradiate an object with reflected light from each of the micromirrors to form a light image on the object.
Originally, the DMD has been proposed for applications in a display such as a digital large screen projector or the like. Recently, however, applications proposed for the DMD have been increasingly directed to an optical writing system for writing a laser beam onto a photosensitive material in an image forming apparatus such as an electrophotographic printer, as described in, for example, Laid-open Japanese Patent Applications Nos. 8-318641, 7-232458 and 9-300701. Particularly, the Application No. 8-318641 describes a method of forming all pixels in the axial direction of a photosensitive material using a DMD having a small number of mirrors and a reduced device length in the axial direction (longitudinal direction). Specifically, the DMD is composed of a plurality of mirror columns arranged in a direction in which the surface of a photosensitive material, as an object, is moved, wherein each of the mirror columns includes a plurality of mirrors arranged in an axial direction, orthogonal to the surface moving direction, and effective reflecting surfaces of mirrors in each mirror column are shifted in the axial direction, so that one line portion of a light image on a photosensitive material is formed using the plurality of mirror columns of the DMD.
In the method disclosed in the above-mentioned Laid-open Japanese Patent Application No. 8-318641, the object is irradiated only with a portion of reflected light from each mirror in order to prevent reflected light images of the respective mirrors from overlapping with each other in the direction orthogonal to the direction of the relative movement of the object, so that the irradiating light having only a reduced intensity reaches each of pixels on the object (photosensitive material). This also leads to a requirement for a technique of irradiating respective pixels with reflected light from the mirrors a plurality of times.
On the other hand, while the pixel density tends to be higher in the field of modern image forming apparatus or the like, the present inventors have found that the pixel density can be significantly improved in the formation of a light image on an object such as a photosensitive material even without the need for increasing the number of mirrors in the direction orthogonal to the direction of the relative movement of the object in a spatial light modulator such as the above-mentioned DMD.